1. Field of the Invention
The present invention relates to active antenna oscillators, and in particular relates to an oscillator made by integrating an antenna with a gain medium. In more detail, the invention relates to an oscillator having an integrated structure emitting high-frequency (30 GHz to 30 THz) electromagnetic waves ranging from millimeter waves to terahertz waves.
2. Description of the Related Art
Recently, non-destructive inspection techniques have been developed using high-frequency electromagnetic waves (may also be called as terahertz waves in the specification) with an arbitrary band wavelength selected from millimeter waves to terahertz (30 GHz to 30 THz) waves. The terahertz waves are known to have absorption lines for absorbing various substances such as biological molecules. The field of application of this frequency range includes an imaging technique that is a safer inspection than the use of X-rays. It also includes a spectral technique for inspecting a molecule integrated state by obtaining an absorption spectrum and a complex dielectric constant in a substance. Moreover, an analytical technique of biological molecules and a technique for estimating a carrier density and mobility are expected.
In these development techniques, the generating technique of terahertz waves is important. The generating technique includes a photoconduction element having a femtosecond laser excitation source and a parametric oscillator having a laser excitation source and using a non-linear optical crystal. There are also a backward wave oscillator (BWO) and a gyrotron oscillator both including a large electron beam accelerator, such as a small electron tube and free electron laser. However, these oscillators utilizing the generating techniques generally have a large device scale.
On the other hand, there are small oscillators utilizing semiconductor techniques, such as a Gunn diode and a resonant tunneling diode (RTD). These oscillators utilize a negative differential resistance (NDR) obtained by the movement or transition of the electron in the semiconductor due to the electric current injection. The oscillation state is achieved by configuring the oscillating circuit including these semiconductor elements as well as by appropriately adjusting the load resistance and the phase in regard to a desired frequency region.
The electromagnetic waves obtained in such a manner may be frequently radiated outwardly by coupling the waves to a radiation device such as an antenna. However, in the high-frequency region, it is difficult to outwardly radiate the electromagnetic waves efficiently because of the propagation loss of the electromagnetic waves and the mismatching between individually designed elements. Then, the antenna element is assumed to be part of the load resistance constituting an oscillating circuit, so that the integral designing and building up have been attempted (see IEEE Transaction on Microwave Theory Tech., vol. 42, pp. 734 to 741, 1994).
The above non-patent document relates to a micro-strip line (MSL) type antenna oscillator in that a patch antenna is connected to a Gun diode formed in the film thickness direction of a dielectric part constituting the micro-strip line. In this antenna oscillator, the patch antenna is used as part of a load resistance for satisfying the oscillation start conditions in equations (1) and (2) shown below. In order to satisfy phase conditions, a phase adjusting stub is used. The patch antenna and a circuit constituted of another strip line are connected together with an impedance conversion circuit. The antenna oscillator has a planar accumulated configuration as a whole.Impedance real part Re: [Yact+Yload]<0   (1)Impedance imaginary part Im: [Yact+Yload]=0   (2)
In equations (1) and (2), “Yact” and “Yload” correspond to an admittance of a gain element (Gun diode) and an admittance of a strip line type oscillation circuit including an antenna, respectively. In this specification, a system, in which a circuit including a part having an electromagnetic wave gain function like a Gunn diode and an antenna are arranged or accumulated, is called as an active antenna. In such active antennas, the active antenna designed and manufactured as an oscillator is often called an active antenna oscillator.
In the related art described above, the oscillating circuit including the antenna is connected to a gain part in parallel therewith to form an oscillator. In a high-frequency circuit, with decreasing wavelength, the scale of the circuit is generally reduced. As a result, it becomes difficult to apply a sufficient load resistance to the antenna element used as part of the load resistance. More specifically, with decreasing wavelength, the value of resistance of the antenna element is reduced. Since the antenna element is connected to the gain element in parallel therewith, in accordance with the decrease in value of resistance of the antenna element, the value of “Yload” is increased. Consequently, in the terahertz wave region, it becomes difficult to satisfy the impedance real part condition (1), so that the oscillator operation is liable to be unstable. Also, depending on the wavelength, the oscillation itself is difficult.
In regard to the waveguide, the propagation loss of electromagnetic waves due to the effect of the boundary surface to the metal is increased in the terahertz wave region. Thus, when a strip line is in heavy usage for an impedance conversion circuit and the phase matching as in the related art described above, the propagation loss of electromagnetic waves becomes large, which may result in deteriorating the generating efficiency of the electromagnetic waves.